A bath condenser-reboiler induces indirect heat exchange between liquefied oxygen from the bottom of a low-pressure distillation column (hereafter referred to as “the low-pressure column”) and nitrogen gas from the top of a high-pressure distillation column (hereafter referred to as “the high-pressure column”) in a cryogenic air separation unit by a double column system. As a result of this process, the bath condenser-reboiler generates a rising gas in the low-pressure column through evaporation and gasification of a portion of the liquefied oxygen, and generates a reflux liquid in both columns through condensation and liquefaction of the nitrogen gas.
Common among such bath condenser-reboilers are those that use plate-fin heat exchanger cores. These plate-fin heat exchanger cores have large numbers of heat exchange passages formed from adjacent condensation passages and evaporation passages via parting sheets, and are immersed in a liquid bath, and are formed such that the condensation fluid (nitrogen gas) introduced as a gas undergoes condensation and liquefaction in the condensation passages by indirect heat exchange with the evaporation fluid (liquefied oxygen) in the liquid bath and flows downward through the heat exchanger core, while a portion of the liquefied oxygen introduced into the evaporation passages from the bottom of the heat exchanger core undergoes evaporation and gasification and flows upward trough the heat exchanger core.
The flow into the evaporation passages from bottom and the upward flow occur because evaporation causes the liquid density to be lower than the density inside the liquid bath (a thermosiphoning effect), but because the heat exchanger core is used fully immersed in liquefied oxygen, the liquid head of the liquefied oxygen causes the flow into the heat exchanger core to occur at a lower temperature than the boiling point. The liquid head is expressed as the pressure converted to a liquid height. Accordingly, not only is a certain core height required until boiling begins, but the increase in temperature to the boiling point means the temperature difference with the nitrogen gas of the condensed fluid cannot be ensured, causing the pressure of the nitrogen gas to rise and increasing operating costs.
To resolve this problem caused by the liquid head of the liquefied oxygen, Patent Document 1 discloses a multistage bath condenser in which the evaporation zone is partitioned vertically into multiple zones, and multiple stages of liquid reservoir are provided to hold the liquefied oxygen in each evaporation zone, thereby reducing increases in boiling point and improving efficiency. When the liquid reservoir is provided in multiple stages, means for connecting the liquid reservoirs in each evaporation zone and supplying liquefied oxygen to each liquid reservoir are required. In relation to this point, in Patent Document 1, as illustrated in FIG. 1 and FIG. 4 of Patent Document 1, liquid reservoir sections to hold liquid are provided on one or both surfaces in the width direction of the heat exchanger core, and means for connecting the liquid reservoir sections so that the liquid can be supplied to each liquid reservoir are provided.